I've seen an original Stilo subwoofer for sale on eBay that I'm tempted to go for!

Problem is mine didn't come with one obviously so I doubt I have the wiring for it, so is this something that can be easily fitted either with oem wiring or aftermarket?

Mark

The power supply for the combined amplifier/subwoofer may already be in the rear fuse box.

You would need to disconnect all the wires from your speakers, then extend them and route them to the amplifier inputs, and route new wires from the amplifier outputs to your speakers.
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Yeah it wouldn't be a problem if the built in amp didn't power the cabin speakers aswell that way you could easily wire it in to your existing system. Your probably better off buying a sub enclosure and just wiring it in the your head unit if you really want boom boom tunes ........ That's what I did anyway

Here's my homemade effort

Thanks guys, seems a bit much to wire in the standard amp and sub plus I've read they aren't really that great to begin with? I'm toying with the idea of a good quality boot build that, when time and funds allow, I can add upgraded speakers and tweeters up front and also a new double din LCD screen with custom facia I own a dog but she's at the age where I can no longer take her in the car to the likes of the coast, so I want to make use of the boot space that's more to my liking

Let me get this right - a sub can be wired straight to the HU but an amp has to be connected between the input and outputs of the speakers?

Not quite, the sub has to be wired to the head unit via an amp. Because of the sheer size and power out puts of even a small 8 inch sub, an amp of a suitable power rating has to be used as the power out put of a head unit won't be enough to power a sub

So basically you run RCA leads from your head unit to the boot, along with a power lead (preferably from the battery) and a remote power lead, plug them into your chosen amp then run your subwoofer wires from the amp to the sub box, then "Roberts your fathers brother" you have BOOM BOOM